To avoid recursive faults, stack overflow detection has to be
performed before writing in the stack in exception prologs.

Do it by checking the alignment. If the stack pointer alignment is
wrong, it means it is pointing to the following or preceding page.

Without VMAP stack, a stack overflow is catastrophic. With VMAP
stack, a stack overflow isn't destructive, so don't panic. Kill
the task with SIGSEGV instead.

A dedicated overflow stack is set up for each CPU.

  lkdtm: Performing direct entry EXHAUST_STACK
  lkdtm: Calling function with 512 frame size to depth 32 ...
  lkdtm: loop 32/32 ...
  lkdtm: loop 31/32 ...
  lkdtm: loop 30/32 ...
  lkdtm: loop 29/32 ...
  lkdtm: loop 28/32 ...
  lkdtm: loop 27/32 ...
  lkdtm: loop 26/32 ...
  lkdtm: loop 25/32 ...
  lkdtm: loop 24/32 ...
  lkdtm: loop 23/32 ...
  lkdtm: loop 22/32 ...
  lkdtm: loop 21/32 ...
  lkdtm: loop 20/32 ...
  Kernel stack overflow in process test[359], r1=c900c008
  Oops: Kernel stack overflow, sig: 6 [#1]
  BE PAGE_SIZE=4K MMU=Hash PowerMac
  Modules linked in:
  CPU: 0 PID: 359 Comm: test Not tainted 5.3.0-rc7+ #2225
  NIP:  c0622060 LR: c0626710 CTR: 00000000
  REGS: c0895f48 TRAP: 0000   Not tainted  (5.3.0-rc7+)
  MSR:  00001032 <ME,IR,DR,RI>  CR: 28004224  XER: 00000000
  GPR00: c0626ca4 c900c008 c783c000 c07335cc c900c010 c07335cc c900c0f0 c07335cc
  GPR08: c900c0f0 00000001 00000000 00000000 28008222 00000000 00000000 00000000
  GPR16: 00000000 00000000 10010128 10010000 b799c245 10010158 c07335cc 00000025
  GPR24: c0690000 c08b91d4 c068f688 00000020 c900c0f0 c068f668 c08b95b4 c08b91d4
  NIP [c0622060] format_decode+0x0/0x4d4
  LR [c0626710] vsnprintf+0x80/0x5fc
  Call Trace:
  [c900c068] [c0626ca4] vscnprintf+0x18/0x48
  [c900c078] [c007b944] vprintk_store+0x40/0x214
  [c900c0b8] [c007bf50] vprintk_emit+0x90/0x1dc
  [c900c0e8] [c007c5cc] printk+0x50/0x60
  [c900c128] [c03da5b0] recursive_loop+0x44/0x6c
  [c900c338] [c03da5c4] recursive_loop+0x58/0x6c
  [c900c548] [c03da5c4] recursive_loop+0x58/0x6c
  [c900c758] [c03da5c4] recursive_loop+0x58/0x6c
  [c900c968] [c03da5c4] recursive_loop+0x58/0x6c
  [c900cb78] [c03da5c4] recursive_loop+0x58/0x6c
  [c900cd88] [c03da5c4] recursive_loop+0x58/0x6c
  [c900cf98] [c03da5c4] recursive_loop+0x58/0x6c
  [c900d1a8] [c03da5c4] recursive_loop+0x58/0x6c
  [c900d3b8] [c03da5c4] recursive_loop+0x58/0x6c
  [c900d5c8] [c03da5c4] recursive_loop+0x58/0x6c
  [c900d7d8] [c03da5c4] recursive_loop+0x58/0x6c
  [c900d9e8] [c03da5c4] recursive_loop+0x58/0x6c
  [c900dbf8] [c03da5c4] recursive_loop+0x58/0x6c
  [c900de08] [c03da67c] lkdtm_EXHAUST_STACK+0x30/0x4c
  [c900de18] [c03da3e8] direct_entry+0xc8/0x140
  [c900de48] [c029fb40] full_proxy_write+0x64/0xcc
  [c900de68] [c01500f8] __vfs_write+0x30/0x1d0
  [c900dee8] [c0152cb8] vfs_write+0xb8/0x1d4
  [c900df08] [c0152f7c] ksys_write+0x58/0xe8
  [c900df38] [c0014208] ret_from_syscall+0x0/0x34
  --- interrupt: c01 at 0xf806664
      LR = 0x1000c868
  Instruction dump:
  4bffff91 80010014 7c832378 7c0803a6 38210010 4e800020 3d20c08a 3ca0c089
  8089a0cc 38a58f0c 38600001 4ba2d494 <9421ffe0> 7c0802a6 bfc10018 7c9f2378
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/1b89c121b4070c7ee99e4f22cc178f15a736b07b.1576916812.git.christophe.leroy@c-s.fr